Water damage and rot is the most important thing to check for. Especially in the roof, second in the floor and rest of the unit.
Of second importance is canvas damage from mice and mildew (putting it away wet).
Canvas replacements are available, but you're looking at $700-$1300 depending on the unit. Factor that into the price if the one you're looking at needs it, you might get a good deal on a fixer-upper that's solid other than canvas. The canvas is very easy to replace yourself.
Some campgrounds with bear problems may not allow popups, or will make you store your food in a bear box (provided). I liked our popup, I'm just getting too old to go through the setup process and crank the think up and down several times. Once partway up to load it, another at camp, another at home to unload and let it dry out.
